Interpretive Summary: Increasing the amount of soil organic matter (SOM) improves the ability of soils to supply air, water, and nutrients to growing plants, and may help mitigate global warming by removing greenhouse gasses from the atmosphere. The addition of new SOM to soils through plant decomposition has been widely studied; however, the influence of soil clay minerals on the formation of new SOM is not well understood. We discovered that new SOM preferentially forms on, or accumulates on, the surfaces of a highly reactive type of clay minerals known as smectites. Smectites are the dominant type of clay mineral in the fine clay fractions of many soils. Our results also showed that only a small amount of new SOM accumulates in the coarse clay fraction. The coarse clay fraction is dominated by quartz and other less reactive minerals. Because, the coarse clay fraction and the fine clay fraction of soils have about the same amount of SOM, our results imply that the SOM which is in the coarse clay fraction is both older and more stable than the SOM which is in the fine clay fraction. In the short term, the results of this study are important for scientists who are studying how SOM is formed and who are trying to develop management strategies that increase SOM. In the long term, this study may help to identify soils with the greatest potential for building new SOM. This information will help action agencies design agricultural subsidy programs that help mitigate global warming and enhance the quality of agricultural soils.

Technical Abstract:
An understanding of organic C dynamics in soils is necessary to develop management options to enhance soil organic C sequestration. The objective of this research was to study the distribution of newly formed humic materials into mineralogically distinct clay-size fractions of a silt loam soil. Oats (Avena sativa L.), grown under simulated no-tillage conditions, were pulse labeled with **14CO2. After senescence, the labeled surface residue was removed and replaced with unlabeled residue and the labeled roots were allowed to decompose in the soil for 360 days. The soil clay fraction (<2 um) was separated into coarse, medium, and fine clay size-fractions (0.2-2.0, 0.02-0.2 and <0.02 um, respectively) by centrifugation. X-ray diffraction indicated that quartz, illite, and kaolinite were the dominant mineral phases in the coarse clay fraction while smectite was the dominant mineral phase in the fine clay fraction. The organic C content in the coarse and fine clay fractions (3.70 and 3.93%, respectively) was similar. Scintillation analysis indicated an increase in **14C specific activity in all fractions after 360 days of incubation. For both sampling times, 0- and 360-days, the highest **14C specific activity occurred in the fine clay fraction (847.2 and 1529 Bq g**-1 C, respectively), whereas the lowest **14C specific activity occurred in the coarse clay fraction (565.8 and 770.9 Bq g**-1 C, respectively). The results suggest that new humic materials are preferentially forming or accumulating on smectite surfaces.